On the evening of February 23, 2010, Rodney Koon was pulled over for doing 83 in a 55 zone outside Traverse City, Michigan. The deputy who stopped Koon noticed that the inside of his Toyota RAV4 smelled funny, and the middle-aged carpenter admitted that he'd taken a few hits of marijuana six hours earlier.

As a pipe-carrying medical-marijuana user (for a hernia and rheumatoid arthritis), Koon thought that the law was on his side. The cop thought otherwise and took him in for a blood test, which revealed traces of THC, or tetrahydrocannabinol, pot's psychoactive ingredient. Koon was charged with violating the state's "zero tolerance" drugged-driving law. He's still fighting the charges; if convicted, he faces a suspended license and, since he has a previous DUI, up to a year in prison.

Koon's case centers on a contentious practical issue that has emerged with the spread of pot legalization: how to define—and prevent—stoned driving. Between 2009 and 2011, the number of THC-positive blood samples obtained from drivers by Colorado police more than doubled. A study found that about 7 percent of California drivers surveyed on two weekend nights last summer tested positive for THC. Nearly the same percentage had alcohol in their blood; 1 percent had both pot and booze. (Drivers who agreed to participate in the study were given legal immunity, handed $20, and hooked up with a ride home if necessary.) But just how stoned is too stoned to drive?
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Ten states, many of which have legalized medical marijuana, simply make it illegal to drive with any trace of marijuana in your blood. Other states essentially regulate the drug like alcohol, requiring drivers to stay below a set limit of cannabinoids in their blood. When Washington voters legalized pot last November, they also outlawed driving with a blood THC level over 5 nanograms per milliliter—about half the level detected in Koon. Ohio and Nevada's limit is even stricter: 2 ng/ml. These rules appeal to a public accustomed to drunk-driving standards, and they give police a simple benchmark for making arrests.